In recent years, base stations called femto base stations have been known. Examples of femto base stations include base stations that form femto cells covering smaller ranges than those of the macrocells of macro base stations and those of the picocells of pico base stations. Femto base stations are used, for example, to provide services for users in environments where it is difficult for radio waves to reach, such as in basements or buildings.
When femto base stations are used, they are in some cases installed at indoor spots in shopping malls and stations where a large number of general users are present. In such cases, a large number of mobile stations (may be referred to as user equipment(s) (UE(s)) or terminal(s)) may concentrate temporarily in the femto cell of a certain femto base station, thereby sharply increasing the load on that femto base station.
However, from the viewpoint of apparatus size and communication load, the number of mobile stations that are connectable to the femto cell of each femto base station (namely, the number of mobile stations that can be served or managed by the femto cell of each femto base station) is smaller than that of macro base stations and pico base stations. Consequently, when a large number of mobile stations concentrate temporarily in the femto cell of a certain femto base station, it is difficult for each mobile station to connect to the femto base station.
To address this difficulty, there is a known connection system that connects the mobile stations to the femto cell in the order that connection requests (may be referred to as attach request(s)) from the mobile stations (that have not be served or managed by the femto cell) arrive at the femto base station. In the connection system, the femto base station monitors the number of mobile stations that are currently connected to the femto cell thereof and rejects a connection request from a new mobile station when the number of connected mobile stations reaches a pre-defined threshold. This arrangement guarantees communication of mobile stations that are currently connected to a certain base station, when a large number of mobile stations concentrate in the cell of the base station.
Related art is disclosed in Japanese Laid-open Patent Publication No. 2011-182009 and Japanese Laid-open Patent Publication No. 2010-283440.